Nonwoven fabrics are widely used in a variety of products. For example, nonwoven fabrics are suitable for use in filters, roofing materials, composites, backing materials, linings, insulation, medical/surgical applications, bedding, tablecloths, and diapers. High loft batting nonwoven fabrics are also used in a wide variety of products, including comforters, robe wear, and bra cups. Generally nonwoven fabrics are based on polyester, acrylic, nylon, glass and cellulosic fibers which may be bonded with latex adhesives, binder fibers, or polymers in powder form. The bonding of nonwoven fabrics with binder fibers provides a convenient method for making nonwoven fabrics without the need for water-based adhesives which are less environmentally friendly. Nonwoven fabrics bonded with binder fibers are economical to produce, and provide a method for making articles, which are unique or superior in performance.
Certain copolyesters have been found to be useful as binder fibers. For example, polyethylene terephthalate (PET) copolyesters containing 1,4-cyclohexanedimethanol having inherent viscosity (I.V.) values in the range of about 0.6 to about 0.8 have been used in the past as binder fibers to bond polyester or other fibers. Copolyesters with lower I.V. values, however, were believed to not have adequate bonding strength.
It is well known that copolyesters can be prepared by processes involving polyesterification and polycondensation. Generally, as described in U.S. Pat. Nos. 2,901,466, 5,017,680, 5,106,944, 5,668,243 and 5,668,243, the reactants are a glycol component and a dicarboxylic acid component. Typically, the dicarboxylic acid component is terephthalic acid and the dihydric alcohol is ethylene glycol. Such copolyesters are relatively inert, hydrophobic materials which are suitable for a wide variety of uses, including, molded articles, food trays, fibers, sheeting, films and containers, such as bottles. The use of ethylene glycol as the only diol, however, is accompanied by undesirable properties such as yellow discoloration and weak fiber binding properties. Indeed, such polymers tend to be opaque, crystalline polymers with high melting temperatures which make them unsuitable for use as binder fibers. To remedy the problems with polyethylene terephthalates, polyethylene terephthalate copolyesters have been formed with 1,4-cyclohexanedimethanol.
The preparation of copolyesters with ethylene glycol, 1,4-cyclohexanedimethanol and terephthalic acid is typically conducted in the presence of a catalyst materials. The choice of materials for such have generally focused on a variety of combinations of materials including catalysts derived from antimony, cadmium, calcium, gallium, germanium, lithium, magnesium, manganese, titanium, and zinc. An exemplary catalyst system for the preparation of polyethylene terephthalate is described in U.S. Pat. No. 3,907,754. Unfortunately, previous catalyst systems have not been entirely successfiil as they can produce copolyesters having an undesirable discoloration. Thus, there exists a need in the art for a catalyst system which provides efficient reaction times while producing high clarity copolyesters, especially high clarity copolyester binder fibers.
Furthermore, previous attempts at forming copolyesters with 1,4-cyclohexanedimethanol have focused upon copolyesters having high inherent viscosities, I.V. of greater than 0.6, due to the beliefs that low inherent viscosities would not possess adequate strength. In particular, it was believed that low inherent viscosity copolyesters were unable to provide adequate bonding strength to form commercially acceptable binder fibers. Indeed, previous polyethylene terephthalate copolyesters having 1,4-cyclohexanedimethanol were made with inherent viscosities ranging from 0.6 to 0.8 to form binder fibers to bond polyesters or other fibers. However, such attempts have not been entirely successful in providing copolyesters having the desired high clarity and hue or bonding capability at low activation temperatures when in the form of a binder fiber. Thus, there exists a need in the art for a copolyester having an inherent viscosity of less than 0.6 while possessing improved clarity, color and binder fiber bonding strength at low activation temperatures.